1. Field of Invention
The invention concerns a method for rapidly and automatically making and supplying at least one denture on the basis of digital image information of a part or all of the human mouth, according to which this image information of the mouth is directly converted, after digital transmission via phone or internet, by means of the rapid prototyping technique and thus with a computer processing unit and a rapid prototyping machine, into a working usable disposal plastic denture for immediate use in the human mouth.
2. Prior Art
Dentures herein are defined as full or partial dentures. A partial denture is defined herein as one juxtaposed to a natural or implanted tooth. A full denture may consist of either a denture overfitting either the upper gums with all teeth removed or the lower gums with all teeth removed. Partial dentures as such are beyond the scope of this disclosure for technical, physical, economic, and other considerations inconsistent with an essential objection of the invention, namely, to make full dentures and replacements thereof inexpensive, rapidly obtainable, and disposably made of a plastic material comparable to the transition in the contact lens industry from permanent glass contacts to inexpensive disposable plastic contacts.
By rapid prototyping technique should be understood all techniques whereby an object is built layer by layer or point per point by adding or hardening material (also called free-form manufacturing). The best known techniques of this type are: stereo lithography and related techniques, whereby for example a basin with liquid synthetic material is selectively cured layer by layer by means of a computer-controlled electromagnetic beam; selective laser sintering, whereby powder particles are sintered by means of an electromagnetic beam or are welded together according to a specific pattern; or fused deposition modeling, whereby a synthetic material is fused and is stacked according to a line pattern. A computer tomography scanner can provide the digital image information.
The model produced up to now according to the above-mentioned technique, can be a dental crown or other partial dental restoration such as an inlay that is an exact copy of the part as digitally stored in a computer, or it can be a prosthesis that fits perfectly to a matching part in the part of the body.
However, the dentures produced up to now, including three-dimensional images, do not take advantage of lower costs of delivering a finished upper and/or lower or full mouth denture directly to the consumer using rapid modeling or prototyping technology for mass production of these related but individualized dentures using robotics, computers, CAD programs, and modern plastics to substantially reduced the cost, eliminate the need for dentists, and deliver the finished denture in multiple disposable plastic sets to a consumer. Moreover, the dentures available and made today exhibit exterior styling and tooth structure that have no bearing or relationship to the user's teeth size and arrangement before their teeth were completely removed and the fact that digital storage technology and laser scanning now allows much wider latitude in denture style at lower cost and faster turnaround time.
Models for prostheses which are exact copies of real structures have been, for example, produced from medical images with the technique disclosed in the article “Integration of 3-D medical imaging and rapid prototyping to create stereo lithographic models” from T. M. BARKER et al., published in “Australasian Physical & Engineering Sciences in Medicine”, vol. 16, no. 2, June 1993, pages 79–85.
Scanner data are transformed to a suitable format in a computer and the images are processed as a volume of voxels. The object is segmented prior to the meshing of the object surface and the creation of the stereo lithographic model. The obtained model cannot be used for registration, this is finding back a position on the patient.
As for the application of dental implants, attempts have already been made to use teeth of a provisional prosthesis as a reference. This provisional prosthesis is made on the basis of a mould. With a reconstruction by means of computer tomography scanner images on the basis of planes in which the bone is clearly visible, what is called a dental scan, one can see whether the position and the angle of the provisional teeth are correct in relation to the underlying bone, and one can make corrections. However, this is a time-consuming method and costly method employing tomography in a way different from the instant novel invention. Another prior art method consists in making a model of the jaw by means of the rapid prototyping technique and to make a template on the basis of this model, which is used during the surgery.
Heretofore, dental practice has been slow to address the need to quickly provide full dentures to the elderly and others needing it. The emphasis of the novel invention presented here is on the practicality of bringing a quick, relatively inexpensive product into the marketplace for delivery to people replacing original dentures or otherwise toothless with no obstructions.
Dentists are charging around $75.00 per tooth in preparation for a denture. For the denture itself, dentists are charging fees in the neighborhood of about $1400 or more. In major cities, according to some interviewed dental practitioners, it takes 2 weeks start to finish—$75 per tooth extraction, and an additional $1400 to $2400 for the dentures. Current full dentures centers all around the country offer so-called quickness at 2 weeks as opposed to maybe 6 to 8 weeks with an established dentist who of course does many other dental procedures.
In 1998 related U.S. Pat. No. 5,768,134 was issued to Swaelens et al. for a method for making a perfected medical model on the basis of digital image information of a part of the body. According to which this image information of a part of the body is converted, by means of what is called the rapid prototyping technique and thus with a processing unit and a rapid prototyping machine, into a basic model of which at least a part perfectly shows the positive or negative form of at least a portion of the part of the body. At least an artificial functional element with a useful function is added to the basic model as a function of the digital information and possibly as a function of additional external information.
In 1996 related U.S. Pat. No. 5,557,297 was issued to Hyde et al. for an aircraft based topographical data collection and processing system for rapidly and accurately determining the topography of land masses as well as individual x, y, z coordinates of discrete objects and/or terrain. This patent is hereby incorporated herein by reference.
In 1993 related U.S. Pat. No. 5,189,781 was issued to Weiss et al. for a rapid tool manufacturing method requiring first building an SFF pattern made of plastic. Rapid prototyping machinery currently available include a Helisys model LOM 1015, a 3D System's model SLA250 and Stratasys's model FDM-2000. These machines are complicated and do require regular service.
The term rapid prototyping (RP) refers to a class of technologies that can automatically construct physical models from Computer-Aided Design (CAD) data. These “three dimensional printers” allow designers to quickly create tangible prototypes of their designs, rather than just two-dimensional pictures. Such models have numerous uses.
In addition to prototypes, RP techniques can also be used to make tooling (referred to as rapid tooling) and even production-quality parts (rapid manufacturing). For small production runs and complicated objects, rapid prototyping is often the best manufacturing process available. Of course, “rapid” is a relative term. Most prototypes require from three to seventy-two hours to build, depending on the size and complexity of the object. This may seem slow, but it is much faster than the weeks or months required to make a prototype by traditional means such as machining. These dramatic time savings allow manufacturers to bring products to market faster and more cheaply. In 1994, Pratt & Whitney achieved “an order of magnitude cost reduction and time savings of 70 to 90 percent” by incorporating rapid prototyping into their investment casting process.
At least six different rapid prototyping techniques are commercially available, each with unique strengths. Because RP technologies are being increasingly used in non-prototyping applications, the techniques are often collectively referred to as solid free-form fabrication, computer automated manufacturing, or layered manufacturing. The latter term is particularly descriptive of the manufacturing process used by all commercial techniques. A software package “slices” the CAD model into a number of thin (e.g. 0.1 mm) layers, which are then built up one atop another. Rapid prototyping is an “additive” process, combining layers of paper, wax, or plastic to create a solid object.
In contrast, most machining processes (milling, drilling, grinding, etc.) are “subtractive” processes that remove material from a solid block. RP's additive nature allows it to create objects with complicated internal features that cannot be manufactured by other means. Of course, rapid prototyping is not perfect. Part volume is generally limited to 0.125 cubic meters or less, depending on the RP machine. Metal prototypes are difficult to make, though this should change in the near future. For metal parts, large production runs, or simple objects, conventional manufacturing techniques are usually more economical. These limitations aside, rapid prototyping is a remarkable technology that is revolutionizing the manufacturing process.
The Basic Process
Although several rapid prototyping techniques exist, all employ the same basic five-step process. The steps are:
Create a CAD model of the design
Convert the CAD model to STL format
Slice the STL file into thin cross-sectional layers
Construct the model one layer atop another
Clean and finish the model
First, the object to be built is modeled using a Computer-Aided Design (CAD) software package. Solid modelers, such as Pro/ENGINEER, tend to represent 3-D objects more accurately than wire-frame modelers such as AutoCAD, and will therefore yield better results. The designer can use a pre-existing CAD file or may wish to create one expressly for prototyping purposes. The various CAD packages use a number of different algorithms to represent solid objects. To establish consistency, the STL (stereo lithography, the first RP technique) format has been adopted as the standard of the rapid prototyping industry. The second step, therefore, is to convert the CAD file into STL format. This format represents a three-dimensional surface as an assembly of planar triangles, “like the facets of a cut jewel.” The file contains the coordinates of the vertices and the direction of the outward normal of each triangle. Because STL files use planar elements, they cannot represent curved surfaces exactly. Increasing the number of triangles improves the approximation, but at the cost of bigger file size. Large, complicated files require more time to pre-process and build, so until now with larger and more powerful computers, the designer had to balance accuracy with manageability to produce a useful STL file.
In the third step, a pre-processing program prepares the STL file to be built. Several programs are available, and most allow the user to adjust the size, location and orientation of the model. Build orientation is important for several reasons. First, properties of rapid prototypes vary from one coordinate direction to another. For example, prototypes are usually weaker and less accurate in the z (vertical) direction than in the x-y plane. In addition, part orientation partially determines the amount of time required to build the model. Placing the shortest dimension in the z direction reduces the number of layers, thereby shortening build time.
The preprocessing software slices the STL model into a number of layers from 0.01 mm to 0.7 mm thick, depending on the build technique. The program may also generate an auxiliary structure to support the model during the build. Supports are useful for delicate features such as overhangs, internal cavities, and thin-walled sections.
The fourth step is the actual construction of the part. Using one of several techniques (described in the next section) RP machines build one layer at a time from polymers, paper, or powdered metal. Most machines are fairly autonomous, needing little human intervention. The final step is post-processing. This involves removing the prototype from the machine and detaching any supports. Some photosensitive materials need to be fully cured before use. Prototypes may also require minor cleaning and surface treatment. Sanding, sealing, and/or painting the model will improve its appearance and durability.
Rapid Prototyping Techniques
Most commercially available rapid prototyping machines use one of six techniques. At present, trade restrictions severely limit the import/export of rapid prototyping machines and technology from the U.S.
Stereo Lithography
Patented in 1986, stereo lithography started the rapid prototyping revolution. The technique builds three-dimensional models from liquid photosensitive polymers that solidify when exposed to ultraviolet light. As shown in the figure below, the model is built upon a platform situated just below the surface in a vat of liquid epoxy or acrylate resin. A low-power highly focused UV laser traces out the first layer, solidifying the model's cross section while leaving excess areas liquid.
Next, an elevator incrementally lowers the platform into the liquid polymer. A sweeper re-coats the solidified layer with liquid, and the laser traces the second layer atop the first. This process is repeated until the prototype is complete. Afterwards, the solid part is removed from the vat and rinsed clean of excess liquid. Supports are broken off and the model is then placed in an ultraviolet oven for complete curing.
Stereo lithography Apparatus (SLA) machines have been made since 1988 by 3D Systems of Valencia, Calif. To this day, 3D Systems is the industry leader, selling more RP machines than any other company. Because it was the first technique, stereo lithography is regarded as a benchmark by which other technologies are judged. Early stereo lithography prototypes were fairly brittle and prone to curing-induced warpage and distortion, but recent modifications have largely corrected these problems.
Laminated Object Manufacturing
In this technique, developed by Helisys of Torrance, Calif., layers of adhesive-coated sheet material are bonded together to form a prototype. The original material consists of paper laminated with heat-activated glue and rolled up on spools. As shown in the figure below, a feeder/collector mechanism advances the sheet over the build platform, where a base has been constructed from paper and double-sided foam tape. Next, a heated roller applies pressure to bond the paper to the base. A focused laser cuts the outline of the first layer into the paper and then cross-hatches the excess area (the negative space in the prototype). Cross-hatching breaks up the extra material, making it easier to remove during post-processing. During the build, the excess material provides excellent support for overhangs and thin-walled sections. After the first layer is cut, the platform lowers out of the way and fresh material is advanced. The platform rises to slightly below the previous height, the roller bonds the second layer to the first, and the laser cuts the second layer. This process is repeated as needed to build the part, which will have a wood-like texture.
In recent years Helisys has developed several new sheet materials, including plastic, water-repellent paper, and ceramic and metal powder tapes. The powder tapes produce a “green” part that must be sintered for maximum strength.
Selective Laser Sintering
Developed by Carl Deckard for his master's thesis at the University of Texas, selective laser sintering was patented in 1989. The technique uses a laser beam to selectively fuse powdered materials, such as nylon, elastomer, and metal, into a solid object. Parts are built upon a platform, which sits just below the surface in a bin of the heat-fusable powder. A laser traces the pattern of the first layer, sintering it together. The platform is lowered by the height of the next layer and powder is reapplied. This process continues until the part is complete. Excess powder in each layer helps to support the part during the build. DTM of Austin, Tex., produces SLS machines.
Fused Deposition Modeling
In this technique, filaments of heated thermoplastic are extruded from a tip that moves in the x-y plane. Like a baker decorating a cake, the controlled extrusion head deposits very thin beads of material onto the build platform to form the first layer. The platform is maintained at a lower temperature, so that the thermoplastic quickly hardens. After the platform lowers, the extrusion head deposits a second layer upon the first. Supports are built along the way, fastened to the part either with a second, weaker material or with a perforated junction. Stratasys, of Eden Prairie, Minn. makes a variety of FDM machines ranging from fast concept modelers to slower, high-precision machines. Materials include polyester, polypropylene, ABS, elastomers, and investment casting wax.
Solid Ground Curing
Developed by Cubital, solid ground curing (SGC) is somewhat similar to stereo lithography (SLA) in that both use ultraviolet light to selectively harden photosensitive polymers. Unlike SLA, SGC cures an entire layer at a time. Solid ground curing is also known as the solider process. First, photosensitive resin is sprayed on the build platform. Next, the machine develops a photo mask (like a stencil) of the layer to be built. This photo mask is printed on a glass plate above the build platform using an electrostatic process similar to that found in photocopiers. The mask is then exposed to UV light, which only passes through the transparent portions of the mask to selectively harden the shape of the current layer.
After the layer is cured, the machine vacuums up the excess liquid resin and sprays wax in its place to support the model during the build. The top surface is milled flat, and then the process repeats to build the next layer. When the part is complete, it must be de-waxed by immersing it in a solvent bath. Cubital America Inc. of Troy, Mich. distributes SGC machines in the U.S. The machines are quite big and can produce large models.
Ink-Jet Printing
Unlike the above techniques, Ink-Jet Printing refers to an entire class of machines that employ ink-jet technology. The first was 3D Printing (3DP), developed at MIT and licensed to Soligen Corporation, Extrude Hone, and others.
Parts are built upon a platform situated in a bin full of powder material. An ink-jet printing head selectively “prints” binder to fuse the powder together in the desired areas. Unbound powder remains to support the part. The platform is lowered, more powder added and leveled, and the process repeated. When finished, the green part is sintered and then removed from the unbound powder. Soligen uses 3DP to produce ceramic molds and cores for investment casting, while Extrude Hone hopes to make powder metal tools and products.
Sanders Prototype of Wilton, N.H. uses a different ink-jet technique in its Model Maker line of concept modelers. The machines use two ink-jets. One dispenses low-melt thermoplastic to make the model, while the other prints wax to form supports. After each layer, a cutting tool mills the top surface to uniform height. This yields extremely good accuracy, allowing the machines to be used in the jewelry industry.
3D Systems has also developed an ink-jet based system. The Multi-Jet Modeling technique uses an array of 96 separate print heads to rapidly produce thermoplastic models. If the part is narrow enough, the print head can deposit an entire layer in one pass. Otherwise, the head makes several passes.
Ballistic particle manufacturing was developed by BPM Inc., which has since gone out of business.
Applications of Rapid Prototyping
Rapid prototyping is widely used in the automotive, aerospace, medical, and consumer products industries. Although the possible applications are virtually limitless, nearly all fall into one of the following categories: prototyping, rapid tooling, or rapid manufacturing.
Prototyping
As its name suggests, the primary use of rapid prototyping is to quickly make prototypes for various purposes. Prototypes dramatically improve communication because most people find three-dimensional objects easier to understand than two-dimensional drawings. Such improved understanding leads to substantial cost and time savings. As Pratt & Whitney executive Robert P. DeLisle noted: “We've seen an estimate on a complex product drop by $100,000 because people who had to figure out the nature of the object from 50 blueprints could now see it.” Effective communication is especially important in this era of concurrent engineering. By exchanging prototypes early in the design stage, manufacturing can start tooling up for production while the art division starts planning the packaging, all before the design is finalized.
Prototypes are also useful for testing a design, to see if it performs as desired or needs improvement. Engineers have always tested prototypes, but RP expands their capabilities. First, it is now easy to perform iterative testing: build a prototype, test it, redesign, build and test, etc. Such an approach would be far too time-consuming using traditional prototyping techniques, but it is easy using RP. In addition to being fast, RP models can do a few things metal prototypes cannot. For example, Porsche used a transparent stereo lithography model of the 911 GTI transmission housing to visually study oil flow. Snecma, a French turbo machinery producer, performed photo elastic stress analysis on a SLA model of a fan wheel to determine stresses in the blades.
Rapid Tooling
A much-anticipated application of rapid prototyping is rapid tooling, the automatic fabrication of production quality machine tools. Tooling is one of the slowest and most expensive steps in the manufacturing process, because of the extremely high quality required. Tools often have complex geometries, yet must be dimensionally accurate to within a hundredth of a millimeter. In addition, tools must be hard, wear-resistant, and have very low surface roughness (about 0.5 micrometers root mean square). To meet these requirements, molds and dies are traditionally made by CNC-machining, electro-discharge machining, or by hand. All are expensive and time consuming, so manufacturers would like to incorporate rapid prototyping techniques to speed the process. Peter Hilton, president of Technology Strategy Consulting in Concord, Mass., believes that “tooling costs and development times can be reduced by 75 percent or more” by using rapid tooling and related technologies.
Rapid tooling can be divided into two categories, indirect and direct.
Indirect Tooling
Most rapid tooling today is indirect: RP parts are used as patterns for making molds and dies. RP models can be indirectly used in a number of manufacturing processes:
Vacuum Casting: In the simplest and oldest rapid tooling technique, an RP positive pattern is suspended in a vat of liquid silicone or room temperature vulcanizing (RTV) rubber. When the rubber hardens, it is cut into two halves and the RP pattern is removed. The resulting rubber mold can be used to cast up to 20 polyurethane replicas of the original RP pattern.
A more useful variant, known as the Keltool powder metal sintering process, uses the rubber molds to produce metal tools. Developed by 3M and now owned by 3D Systems, the Keltool process involves filling the rubber molds with powdered tool steel and epoxy binder. When the binder cures, the “green” metal tool is removed from the rubber mold and then sintered. At this stage the metal is only 70% dense, so it is infiltrated with copper to bring it close to its theoretical maximum density. The tools have fairly good accuracy, but their size is limited to under 25 centimeters.
Sand Casting: An RP model is used as the positive pattern around which the sand mold is built. LOM models, which resemble the wooden models traditionally used for this purpose, are often used. If sealed and finished, an LOM pattern can produce about 100 sand molds.
Investment Casting: Some RP prototypes can be used as investment casting patterns. The pattern must not expand when heated, or it will crack the ceramic shell during autoclaving. Both Stratasys and Cubital make investment casting wax for their machines. Paper LOM prototypes may also be used, as they are dimensionally stable with temperature. The paper shells burn out, leaving some ash to be removed.
To counter thermal expansion in stereo lithography parts, 3D Systems introduced QuickCast, a build style featuring a solid outer skin and mostly hollow inner structure. The part collapses inward when heated. Likewise, DTM sells Trueform polymer, a porous substance that expands little with temperature rise, for use in its SLS machines.
Injection molding: CEMCOM Research Associates, Inc. has developed the NCC Tooling System to make metal/ceramic composite molds for the injection molding of plastics. First, a stereo lithography machine is used to make a match-plate positive pattern of the desired molding. To form the mold, the SLA pattern is plated with nickel, which is then reinforced with a stiff ceramic material. The two mold halves are separated to remove the pattern, leaving a matched die set that can produce tens of thousands of injection moldings.
Direct Tooling
To directly make hard tooling from CAD data is the objective of rapid tooling. RapidTool: A DTM process that selectively sinters polymer-coated steel pellets together to produce a metal mold. The mold is then placed in a furnace where the polymer binder is burned off and the part is infiltrated with copper (as in the Keltool process). The resulting mold can produce up to 50,000 injection moldings. In 1996 Rubbermaid produced 30,000 plastic desk organizers from an SLS-built mold. This was the first widely sold consumer product to be produced from direct rapid tooling.
Extrude Hone, in Irwin Pa., will soon sell a machine, based on MIT's 3D Printing process, that produces bronze-infiltrated PM tools and products.
Laser-Engineered Net Shaping (LENS) is a process being developed at Sandia National Laboratories and Stanford University that will create metal tools from CAD data. Materials include 316 stainless steel, Inconel 625, H13 tool steel, tungsten, and titanium carbide cermets. A laser beam melts the top layer of the part in areas where material is to be added. Powder metal is injected into the molten pool, which then solidifies. Layer after layer is added until the part is complete. Unlike traditional powder metal processing, LENS produces fully dense parts, since the metal is melted, not merely sintered. The resulting parts have exceptional mechanical properties.
Direct AIM (ACES Injection Molding): A technique from 3D Systems in which cores are used with traditional metal molds for injection molding of high and low density polyethylene, polystyrene, polypropylene and ABS plastic. Very good accuracy is achieved for fewer than 200 moldings. Long cycle times (e.g. five minutes) are required to allow the molding to cool enough that it will not stick to the SLA core.
In another variation, cores are made from thin SLA shells filled with epoxy and aluminum shot. Aluminum's high conductivity helps the molding cool faster, thus shortening cycle time. The outer surface can also be plated with metal to improve wear resistance. Production runs of 1000–5000 moldings are envisioned to make the process economically viable.
LOM Composite: Helysis and the University of Dayton are working to develop ceramic composite materials for Laminated Object Manufacturing. LOM Composite parts would be very strong and durable, and could be used as tooling in a variety of manufacturing processes.
Sand Molding: At least two RP techniques can construct sand molds directly from CAD data. DTM sells sand-like material that can be sintered into molds, while Soligen 3D Printing machines can produce ceramic molds as well.
Rapid Manufacturing
A natural extension of RP is rapid manufacturing (RM), the automated production of salable products directly from CAD data. Currently only a few final products are produced by RP machines.
For short production runs RM is very cheap, since it does not require tooling. RM is also ideal for producing custom parts tailored to the user's exact specifications. A University of Delaware research project uses a digitized 3-D model of a person's head to construct a custom-fitted helmet. NASA is experimenting with using RP machines to produce spacesuit gloves fitted to each astronaut's hands.
The other major use of RM is for products that simply cannot be made by subtractive (machining, grinding) or compressive (forging, etc.) processes. This includes objects with complex features, internal voids, and layered structures. Specific Surface of Franklin, Mass. uses RP to manufacture complicated ceramic filters that have eight times the interior surface area of older types. The filters remove particles from the gas emissions of coal-fired power plants.
Therics, Inc. of NYC is using RP's layered build style to develop “pills that release measured drug doses at specified times during the day” and other medical products.
As with the novel invention described below rapid prototyping is starting to change the way companies design and build products. One such improvement is increased speed. “Rapid” prototyping machines are beyond the state of the art by all current standards. By using faster computers, more complex control systems, and improved materials, RP manufacturers are on the verge of dramatically reducing build time. For example, Stratasys recently (January 1998) introduced its FDM Quantum machine, which can produce ABS plastic models 2.5–5 times faster than previous FDM machines.
Today's commercially available machines are accurate to 0.08 millimeters in the x-y plane, but less in the z (vertical) direction. Improvements in laser optics and motor control should increase accuracy in all three directions.
The rapid prototyping industry will continue to grow, both worldwide and at home. The United States currently dominates the field, but Germany, Japan, and Israel are making inroads. In time RP will spread to less technologically developed countries as well. With more people and countries in the field, RP's growth will accelerate further.
One future application is Distance Manufacturing on Demand, a combination of RP and the Internet that will allow designers to remotely submit designs for immediate manufacture. Researchers at UC-Berkeley, among others, are developing such a system.